The recent development of intercalated graphite fibers having electrical conductivities comparable to or greater than copper or silver has given rise to speculation that such lightweight, low cost materials may be able to supplant the heavier, more expensive copper for certain applications. One such application involves the development of lightweight high strength, co-axial cable for the transmission of r.f. power.
In a co-axial cable, because of the skin depth limitations of the conductor at radio frequencies, the size of the inner conductor is limiting. By use of material more highly conducting than copper for the central conductor one can transmit larger power with the same geometry. In addition the intercalated graphite is somewhat lighter and stronger than copper so that considering both lesser weight and smaller losses, this material has an approximate 4 to 1 advantage over copper conductors. To date, the ability to convert graphite into a highly conducting material has been demonstrated. In the method for making wires, graphite powder is intercalated with antimony pentafluoride, a strong Lewis acid, and packed into a copper ampoule 6 mm in diameter and sealed. The ampoule is swaged or drawn into a wire one millimeter in diameter. The result is a composite wire having a sheath of copper and a core of oriented antimony pentafluoride graphite. Measurements indicate that the core has an electrical resistivity at room temperature of 1.times.10.sup.-6 .OMEGA.-cm which compares very favorably with the resistivity of pure copper which is 1.7.times.10.sup.-6 .OMEGA.-cm. While the above method produces a wire of intercalated graphite, its tensile strength is relatively poor because the thin copper sheath has little strength and the compacted fine graphite grains constituting the core cannot support any tensile stress. Furthermore, the thin copper sheath is not in sufficiently intimate contact with the grains of the core to allow heat to be dissipated effectively. This is because these grains are aligned with their conducting planes parallel to the wire axis and since graphite has a large anisotropy in both its electrical and thermal conductivities, heat flow transverse to the planes will be inhibited. For example, the electrical resistivity measured parallel to the hexagonal conducting layer planes is lower than that measured normal thereto by a factor of about 10.sup.6. The same is true of the thermal conductivity. In fabricating the copper-clad composite wire with the intercalated graphite core, it is necessary that the high conductivity directions of the grains be aligned with the wire axis, and fortunately this happens naturally in the wire forming process. But unfortunately, because of the high anisotropy, a large impedance to heat transfer between grains will develop in the radial direction. Thus, the thin copper outer layer is not effective in dissipating I.sup.2 R heat developed in such wires.